The current proposal aims to develop a physical neuroprosthesis system, directly controlled by cortical signals, to restore high-dimension hand functionality to Veterans with chronic tetraplegia resulting from high cervical spinal cord injury (SCI). This unique approach combines an intracortical brain-computer-interface (iBCI) that captures a user?s volitionally intended movements, with a percutaneous functional electrical stimulation (FES) system for activation of paralyzed muscles. While there exist research efforts to use BCIs to control the motion of simple computer cursors and robotic devices, the present proposal will investigate the possibility of restoring high-dimensional multi-grasp control of one?s own paralyzed hand. When realized, this system will enable Veterans with high cervical SCI to cortically command more hand grasp patterns than is realized by current FES neuroprosthetic systems, thereby allowing them to carry out a larger range of everyday grasping tasks. Dexterous hand function is a critical component to being able to perform the object manipulations necessary for successfully completing a wide variety of everyday reaching-and-grasping tasks. For example, the hand grasp required to pick up small flat objects differs from that required to interact with larger cylindrical objects, which differs from the hand grasp required for pointing tasks (e.g. pushing a button). Current clinically relevant neuroprosthetic systems implement one or at most two preprogrammed hand grasp patterns that the user can switch between, though usually not seamlessly. A hand grasp neuroprosthesis that allows users a larger variety of hand grasp patterns by allowing for independent thumb and index finger control, as well as possibly a larger set of switchable preprogrammed grasp patterns, will likely prove functionally advantageous over current clinically available systems. Currently available systems are partly limited in the scope of functionality due to the lack of available signals for commanding the multiple dimensions necessary for increased grasp flexibility. Recording of brain signals directly related to hand function, through an intracortical brain-computer-interface (iBCI) may allow for a richer source of signals for commanding higher dimensions of hand function. This proposal will thus investigate using iBCIs as a command source for implementing a high-dimensional FES hand neuroprostheses, by investigating 1) the possibility of commanding multiple hand grasp types from cortical activity, and 2) the possibility of commanding individual thumb and index movements from cortical activity, thereby allowing the user greater flexibility in creating their own hand grasps. The proposed work will engage Veterans and other participants enrolled in the BrainGate2 Pilot Clinical Trial. As part of the BrainGate2 Pilot Clinical Trial, these participants will already have received a) two intracortical microelectrodes implanted into motor cortical areas controlling arm and hand movements, and b) a percutaneous FES system consisting of small fine-wire electrodes implanted into various muscles of the arm and hand to restore reaching and grasping function through electrical stimulation. Thus this proposal will allow for direct testing of control concepts in Veterans with paralysis. When successful, the proposed work will 1) result in a brain-controlled highly dexterous hand neuroprosthesis for paralyzed Veterans, and 2) offer quantitative insight about the long term potential of iBCIs for commanding complex neuroprosthetic hand function for Veterans considering an intracortical implant for functional grasp restoration after neurological injury or disease.